A variety of optical fiber sensors have come into use. Among them are: a voltage sensor which is a combination of a Pockels cell and an optical-fiber cable; a current sensor which is a combination of a Faraday cell and an optical-fiber cable; and a pressure sensor which is a combination of a photoelastic cell and an optical-fiber cable. Each of these sensors has a detector section connected in a single optical path made of an optical-fiber cable. The detector section comprises a light-modulating element and an optical element connected in series. In the light-modulating element, its birefringence degree changes in accordance with the physical quantity sensed. The polarization state of the light output from the light-modulating element is modulated thereby. The optical element converts the polarization state into the intensity of light. The signals exchanged between the detector section and a device connected to the sensor are exclusively optical signals. Therefore, the optical fiber sensor is greatly insulative and explosion-proof, and can be used in various conditions. It is not only versatile, but also greatly safe.
These optical fiber sensors have yet to be used widely, however. This is because they make large detection errors. Each of these sensors requires a light source. The intensity of the light from the light source is likely to change with temperature or the like. The loss in the optical-fiber cable, and also the loss in the optical connector connective the cable vary slightly even with external factors. Such a change, if occurring, results in a detection error. The optical fiber sensor should therefore be equipped with means for sufficiently compensating for the change mentioned above.
Known as a method of compensating for the drift of a detection signal, which has resulted from the above-mentioned change, is the method disclosed in Ohnishi, ed., Optical Fiber Sensors. Ohm Co., 1986, pp. 131-133. In this method, the AC component of the optical signal generated by a detector section is divided by the DC component of the signal, thereby to compensate for the drift, or the light source is controlled, thereby to adjust the intensity of the output light such that the mean light intensity detected by the detector section is constant, thus compensating for the drift.
This method is effective when the physical quantity to be detected changes alternately, but cannot compensate for the drift when the physical quantity changes linearly.
In another method, two polarized components, whose planes are perpendicular to each other, are supplied via an optical-fiber cable. The difference between these polarized components is divided by the sum of the components, thereby compensating for a drift component. In another known method, two light sources emitting light beams of different wavelengths are used. The light beam emitted from the first light source is supplied to a detector section through the optical-fiber cable, while the light beam emitted from the second light source is passed through an optical-fiber cable used as a dummy. The intensity of the light, detected by the detector section, is divided by the intensity of the light passed through the dummy optical-fiber cable, thereby compensating for a drift component.
Either method described above cannot compensate for a drift component. This is because the light required for the compensation is guided through a system other than the detecting system, and it is difficult to equalize the changes in the loss at the optical-fiber cables and optical connectors of both systems.
Accordingly, the object of the present invention is to provide an optical fiber sensor, which is relatively simple in structure and can reliably compensate for the changes in the intensity of the light emitted from the light source and the changes in the loss at the optical-fiber cable and the optical connector, even if the physical quantity to be sensed varies either alternately or linearly.